1. Field
This disclosure generally relates to the fabrication of composite structures, and deals more particularly with a method and apparatus for forming flat composite charges into reinforcing substructures, such as blade-type stiffeners, including stiffeners that are contoured and/or have a variable gauge.
2. Background
Composite reinforcing substructures such as blade stiffeners, sometimes referred to as blade stringers, are frequently used in the marine and aircraft industries. These stringers may be fabricated by combining two or more stiffening members. For example, a stringer have an I-shaped cross section, sometimes referred to an I-stringer, can be fabricated by combining two members having a C-shaped cross section, back-to-back. These members may be formed by manually hot drape forming multiple composite charges over a mandrel or other tool. After forming, the members are placed back-to-back and co-cured in an autoclave. Fabricating blade stringers using multiple separate charges requires multiple tools, is relatively labor intensive and may add to manufacturing flow times.
In some applications, stiffeners such as the blade stringers mentioned above, may need to be contoured along their lengths in order to conform them to a structure such as a contoured aircraft skin, to which they are to be attached. Difficulty may be encountered, however, when attempting to form highly contoured stringers using conventional tooling because of the tendency of the plies in the charge to wrinkle as the charge is being compressed. Accordingly, fabrication of highly contoured stringers using composites is generally limited to hand layup techniques in which each ply is laid up by hand over a die or other tool in order to reduce the possibility of ply wrinkling. The hand layup technique is labor intensive, and thus costly as well as relatively slow. An added challenge is presented where one or more areas of the stringer include ply drop-offs and/or pad-ups in order to conform the stringer to local contouring of the skin.
Accordingly, there is a need for a method and apparatus for fabricating composite blade stiffeners such as I-stringers, using a single composite charge formed with relatively simple tooling. There is also a need for a method and apparatus of the type mentioned above that may be adapted to allow contouring of the stringer along its length, including localized contouring using ply drop-offs or pad-ups.